<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Document</title>
    <style>
        body{
            background-color: #ccc;
        }
        .box{
            width: 0px;
            height: 0px;
            /* border: 10px solid red; */

            /* border-top: 10px solid red;
            border-right: 10px solid blue;
            border-bottom: 10px solid yellowgreen;
            border-left: 10px solid pink; */


            /* border-top: 10px solid red;
            border-right: 10px solid #fff;
            border-bottom: 10px solid #fff;
            border-left: 10px solid #fff; */


            /* border-top: 10px solid red;
            border-right: 10px solid rgba(0, 0, 0, 0);
            border-bottom: 10px solid rgba(0, 0, 0, 0);
            border-left: 10px solid rgba(0, 0, 0, 0); */


            /* border-top: 10px solid red;
            border-right: 10px solid transparent;
            border-bottom: 10px solid transparent;
            border-left: 10px solid transparent; */


            border-bottom: 10px solid red;
            border-right: 10px solid transparent;
            border-top: 10px solid transparent;
            border-left: 10px solid transparent;
        }
    </style>
</head>
<body>
    <div class="box"></div>
</body>
</html>